When people are in an area where there are continuous loud noises or sound, it is not possible for them to clearly hear sounds that they desire to hear. For example, noise generated by an air conditioner in a living room or noise from outside may often interfere with sounds that people desire to hear.
Such sounds desired to be heard by people may be generated by entertainment or multimedia equipment, such as a television or a radio. Actually, the volume perceived by a listener is a function of noise, and when the noise increases, the listener will perceive a lower volume. If the listener desires to maintain the quality of perceived volume at a constant level, he or she must adjust the volume according to the loudness of the ambient noise.
U.S. Pat. No. 5,907,622, entitled “AUTOMATIC NOISE COMPENSATION SYSTEM FOR AUDIO REPRODUCTION EQUIPMENT,” disclosed a system that produces an audio signal in a listening space. The system comprises a number of elements that may be used for generating a desired volume signal and a noise signal representative of ambient noise conditions in the listening space. The system further comprises a variable gain amplifier for amplifying the audio signal according to a gain control signal, and a device for generating the gain control signal according to a volume control signal. The volume control signal is generated according to the noise and desired volume signal, such that the volume control signal increases in accordance with increases in the noise signal.
From the aforementioned, U.S. Pat. No. 5,907,622 disclosed a way of increasing the gain of the audio signal according to the energy of ambient sound so as to prevent sounds which are desired to be heard from being drowned out by ambient sound. However, this patent has the following drawbacks: since the gain is adjusted according to the volume level of ambient sound, if there is also a significant sound in the ambient sound, the significant sound will be drowned out by audio signals that are increased in gain due to an increase in the ambient sound. In such a situation, people will not take notice of the significant sound.
In United States Patent Application Publication No. 20050069154 A1, entitled “ELECTRONIC APPARATUS THAT ALLOWS SPEAKER VOLUME CONTROL BASED ON SURROUNDING SOUND VOLUME AND METHOD OF SPEAKER VOLUME CONTROL,” when power is turned on, a speaker volume control section is able to obtain the volume of ambient sound from a microphone to determine the output volume of a speaker. The speaker volume control section then displays the value of the output volume of the speaker thus determined on a liquid crystal display (LCD). Subsequently, the speaker volume control section monitors whether or not the user has changed the speaker volume. If the user does not change the speaker volume within a predetermined period of time, the speaker volume control section informs a sound controller of the determined speaker volume. On the other hand, if the user changes the speaker volume, the speaker volume control section informs the sound controller of the volume designated by the user.
From the aforementioned, the patent application publication US 20050069154 was designed to compensate for noise generated when a user was listening to sound, and used a determination unit to determine the value of the output volume of the speaker according to the volume of sound received by the microphone. However, the disadvantage of this patent application resides in the fact that after the gain is applied to the desired listening signal, the microphone receives the ambient sound that also includes the desired listening signal that has been amplified. As a result, in this prior art technique, since the sound volume received by the microphone is increased as the volume of the desired listening signal is increased, it will be determined that the desired listening signal must be amplified further. Therefore, this prior art technique is unable to provide an accurate gain for amplifying desired listening sound signals.